On the base of the common structure of expert system and the simple data collection system, the author expatiate the principle, design and implement of knowledge database, inference engine, explain system and self-learning system of the fault diagnosis expert system. 作者以专家系统的一般结构和简单的数据采集系统为基础,详细阐述了故障诊断专家系统中知识库、推理机、解释机制和自学习机制的原理以及设计实现。